A paper clip is an instrument used to hold a number of sheets of material such as paper and/or film together or removably hold an article to one or more sheets. Paper clips are usually made of a polymer, steel wire, aluminum wire, or copper wire, plastic coated steel, aluminum, or copper wire having elasticity and exhibit torsion when bent to a looped shape. Most paper clips are characterized by the almost two full coplanar loops formed in the wire. When applying the clip to a paper stack, the inner loop is urged either forward or rearward of the outer loop thus creating a gap between the two wherein stacked pieces of paper are placed. The resulting torsion between to two loops tends to bias and return the two loops to a coplanar state, thus removably binding the sheets together. Friction between wire and paper also helps the prevent the sheets from sliding apart. Too many sheets will cause the elastic limit of the material to be exceeded, resulting in permanent deformation.
Commonly used paper clips comprise a length of wire bent to form two coplanar oval loops wherein a long oval loop is concentric with and inside a short oval loop. The oval loops include the longitudinal portions which are parallel to one another. The clips must be held open while paper sheets are inserted into them. The clips engage paper sheets in tension when the paper sheets are inserted forcibly between the loops which separate laterally. An inherent problem in this type of paper clip is the lack of sufficient gripping pressure between the separated wire loops. The gripping pressure is determined by the spring constant of the wire and the thickness of the paper sheets held within. Thicker sheets or more sheets will be held with more pressure than thinner sheets unless the number of sheets becomes too much to be reasonably held inside the paper clip. With even the stronger paper clips, sheets of paper are often likely to fall from a group when a strong enough force is applied which will tend to pull sheets out of the group. In other words, paper clips are often too weak to do the required task of holding pages together.
Paper clips usually have an oblong shape with straight sides, but may also be triangular or circular, or have more elaborate shapes. Moreover, the wire may include grooves, dimples, crimps, ridges, undulations, striations, or simply be smooth rectilinear sections.
Another commonly used method of holding a few sheets of paper together is dog-earring one corner of the pages together. This method is much less dependable than staples or paper clips due to the fact that friction between the pages within the dog-eared corners is the only force holding the pages together.